This invention is in the field of continuous metal-casting machines having a substantially straight or flat moving mold cavity or mold space wherein a casting belt or belts travel from an entrance into and along the mold space to an exit therefrom. The term xe2x80x9csubstantially flatxe2x80x9d herein includes such gentle longitudinal curvature as may assist in keeping a single tensed travelling casting belt against backup means in the moving mold casting space and also includes such gentle transverse curvature as may assist in keeping the belt in firm contact with the surface of metal being solidified in the moving mold space.
Casting belts in continuous casting machines for continuously casting molten metal are formed of suitable heat-conductive, flexible metallic material as known in the art, having a thickness for example in a range from about 0.3 millimeters to about 2 millimeters. Such a belt is revolved under high tensile forces around a belt carriage in an oval path. During revolving, each belt has, in the prior art, continuously passed around a rotating entrance-pulley drum and a rotating exit-pulley drum positioned respectively at entrance and exit ends of the moving mold.
A persistent problem in the use of such machines has been a spatial limitation alongside the inner surface of the casting belt near an entrance region of the casting space where molten metal first contacts the belt as the belt separates from the rotating entrance pulley drum. In the prior art as disclosed in patents of Hazelett et al., referenced above, this spatial limitation can be seen in a side elevation view. This limitation occurs in the form (shape) of a cusp defined between a belt""s inner surface and a downstream half of the rotating entrance-pulley drum in a region where the moving belt tangentially separates from this pulley drum.
In this space-limited xe2x80x9ccusp region,xe2x80x9d precise control of belt distortion is desired because this is the place where very hot incoming molten metal first contacts the moving belt.
A substitute for a rotating entrance-pulley drum was disclosed by Sivilotti et al. in U.S. Pat. Nos. 4,061,178 and 4,061,177. A multiplicity of hydraulic flotation xe2x80x9cspoolsxe2x80x9d defined and supported the belt path. These spools were disclosed using absolute air pressure less than atmosphericxe2x80x94a partial vacuumxe2x80x94to exhaust coolant liquid away from the spools and to force the belt almost against the spools.
Forces associated with such partial vacuum have been found to be insufficient to stabilize casting belts enough to ensure casting of high-quality product. Sivilotti (in U.S. Pat. No. 4,061,177, column 19) disclosed coolant preheated to 40 to 70xc2x0 C. to help stabilize the belts.
However, resulting high partial pressure of water vapor issuing from hot water limited the partial vacuum achievable by Sivilotti et al.
Moreover, water or coolant temperature even at 70xc2x0 C. is too low for adequate belt preheat to enable casting high-quality product.
Yet, coolant temperature at 55 to 70xc2x0 C. (131xc2x0 to 158xc2x0 F.) presents danger of scalding personnel it this hot coolant were to get out of control as through a defective belt or broken conduit.
Consequently, equipment disclosed in these patents did not solve problems of suitably stabilizing a casting belt and ensuring casting of high-quality product.
It is known that smooth solid objects can be xe2x80x9cfloatedxe2x80x9d very close to smooth solid surfaces by means of fluid interposed between them under pressure. However, when one of the objects is flexible and is moving and also is curved, serious problems arise, such as generation of intolerable screeching noises and belt vibrations when attempting to use compressed air for xe2x80x9cfloatingxe2x80x9d a casting belt moving along a curved, stationary support surface.
I have found a non-rotating, fixed, rigid, convex, generally cylindrically curved, levitating xe2x80x9cair pillowxe2x80x9d belt-guiding apparatus which is much less complex than a multiplicity of spools with scalding hot coolant and partial vacuum. Also, I find that this air-pillow apparatus can be devised to overcome or substantially reduce the above problems. The air-pillow apparatus disclosed herein enables an endless, thin-gauge, flexible casting belt in a continuous casting machine to be deflected, curved, or reversed in its course while making available the space formerly occupied in most belt-type machines by the downstream half of the rotating entrance-pulley drum. The space so saved becomes available for improved belt cooling and support apparatus to be employed in this critical zone which includes the above-defined xe2x80x9ccusp regionxe2x80x9d where molten metal first contacts the casting belt.
In a preferred mode of the invention, levitating air (or other gas) is introduced under controlled pressure and volume into a thin, semi-sealed space or spaces between the moving curved inner surface of a casting belt and the convex-curved, generally cylindrical air-pillow apparatus, thereby enabling the casting belt to revolve in its usual path, with only a minimum of friction. In addition, and advantageously, normal belt tension can be applied to the belt during operation.
Preheating a casting belt controls thermally-induced strains in the belt, thereby keeping the belt flat so that the solidifying molten metal being continually cast is protected from disturbance by unpredictable, sudden distortions which otherwise would occur due to thermally-induced strains in the belt where the belt is adjacent to hot metal. Belt preheating enables casting high-quality product. Belt preheating is disclosed in several U.S. Patents assigned to the Assignee of this application.
Flowing room-temperature compressed air against a preheated belt does not much alter its preheat. On the other hand, contact of a hot belt, for example with room-temperature coolant would considerably reduce belt temperature where such coolant contacts the belt. Dry belt preheating, for example by radiant heating, is facilitated by employing the present invention. Among advantages of using dry preheating are those resulting from avoiding use of dangerous, scalding-hot preheating coolant such as in the ""178 arid ""177 patents discussed above. Moreover, using hot water in a room where a casting machine is located will saturate ambient air with water vapor. This air-borne moisture may condense as droplets on casting belts and may cause minor explosions when such droplets are struck by molten metal. Also, high humidity near a casting machine is debilitating on workers performing jobs requiring alertness and continual careful attention, with quick and skilled responses needed for controlling parameters of ongoing continuous casting.